This invention relates to techniques for correcting signals for undesirable components or characteristics. Although this invention finds particular application to multi-element electro-optical imaging systems and, more specifically, to correction for nonuniformities introduced by individual detector elements in an electro-optical imaging system, it will be seen that the invention is useful for providing correction of analog or digital signal trains for any set of undesired characteristics.
It is well known in the art that the numerous elements in a multi-element focal plane array limit system performance due to a lack of uniformity among the individual elements. For example, the responsivity, DC offset variations, and gain variations in a typical infrared detector array give rise to fixed pattern noise of a magnitude which can readily mask low contrast thermal images. In the past, such nonuniformities have been corrected by means of a plurality of background temperature references or by synthesizing at least one such background temperature reference. See, e.g., pp. 4, 298 and 887--"Non-Uniformity Correction in a Multi-element Detector Array," Rode, Nov. 3, 1981.
Analog methods which synthesize the average background temperature for use in correcting for nonuniformities essentially subtract the average response for each detector element from the current response. However, generating a weighted average requires information collection and recording for an initial averaging interval, involving a delay in responsivity for the imaging system. Further, these techniques may require a total reset when the scene environment changes significantly (askewing the weighted average) or when there is no scene change (staring at a blank scene).
Nonuniformity correction has been accomplished in the digital domain by employing digital correction of the digitized video with the terms updated every frame against a thermal reference. In this approach, the digital word corresponding to each picture element is offset and gain compensated individually. However, known digital techniques have proved inefficient where the nonuniformities themselves are several times larger than the desirable signal component, since a portion of the finite dynamic range of the digital system is used in the compensation. The effective dynamic range available for the signal is reduced since the bit precision of the digital signal after compensation will be less than the starting bit precision of the analog-to-digital converter. In addition, techniques using analog feedback of the digitized signal for correction have required the bit size of the digital-to-analog converter to be at least several bits larger than the bit size of the digital word. Unfortunately, high speed digital-to-analog converters larger than 12 bits are not readily available. Even where available, such long bit D/A converters lack linearity and monotonicity over the entire bit range.
Therefore, a need has developed in the art for an improved technique to compensate for nonuniformities in a multi-element detector array in particular and, more generally, an improved technique to correct analog or long length digital signals for undesirable signal characteristics.